Researchers know that ocean acidification can be harmful. The change is caused by an increase in carbon dioxide in ocean water. More CO2 means a more acidic habitat. It can wear away crab shells and fish scales, and it makes it harder for them to grow back.

But what about how acidification is making those species feel?

Martín Tresguerres is a marine biologist based out of the Scripps Institution of Oceanography in California. In a recent study, his team set out to look at what goes on in the brains of juvenile rockfish when they live in acidified waters. He says rockfish have predictable behaviors, so it’s easier to see changes happening in what they do.

“The normal fish, they’re used to moving between the shaded and light parts of the kelp forest,” he says. “For example — looking for food, or interacting with other fish.”

They wanted to see whether acidification would make the fish behave differently. To find out, they looked at certain neurons in the fishes’ brains. Those neurons are known to change what they do when fish has a high blood acid level. In a previous study on Australian clownfish, the change affected sense of smell.

Tresguerres says this study was looking at the same neurological process. For their experiment, they took a rockfish that lived in acidified waters and put it in a tank with two different-colored walls: one light, and one dark.

He says the acidified fish wanted to stay by the dark wall — and so did a fish that had been dosed with an anxiety-inducing drug.

“Ocean acidification affects their neurons in a way that maybe they feel more threatened and they prefer to stay more sheltered,” he says.

It might not sound too significant. But it points to something bigger — acidification was making the fish do the opposite of what they’d normally do.

“Depending on the species, if they normally go offshore at a certain period of time, or they might go to a certain area to spawn and reproduce, it might affect the way they interact with other fish,” he says. “So the potential implications are pretty big.”

Tresguerres’ co-author on the study is Trevor Hamilton, a neuroscientist based out of MacEwan University in Edmonton, Canada. He says the changes they saw would be really long-term in the wild — they looked at the kind of acidification that takes place over a hundred years.

But he says any species of fish in acidified waters is vulnerable to these same effects. Eventually, it could shake up the entire ecosystem.

“What could end up happening is the fish will spend less time leaving their safe environments,” he says. “There is potential for them to get caught by less nets, essentially, and get eaten by less predators. So it could have an effect all the way up the food chain as well as for general fishing for humans.”

And that’s a problem in the Bering Sea. Here, and in other places with colder waters, more carbon dioxide gets absorbed into the ocean, faster. That means these long-term changes might happen sooner.

“So in Alaska, you may see this effect happening a bit sooner, and it may be more pronounced,” he says. “And that would actually be a really, really interesting question to study — lower the water temperature and see what the effect would be.”

That’s part of their next phase of research. Hamilton and Tresguerres hope to do more field work after this. And they want to look at how fish might adapt to the effects of acidification — because that could mean behavioral changes, too. They call it a domino effect.

Hamilton says while this isn’t something that’ll wipe out any species overnight, it’s important for fishermen and regulators to be aware of.

“If this mechanism does actually occur in the future, we will see fish that are more likely to stay closer to their home environment, and explore different situations a lot less,” he says. “We don’t know if it will happen, but it’s something we should definitely be concerned about.”

Hamilton says they picked rockfish for their first study in part because the species is harvested commercially. He says if ocean acidification continues at its current rate, fishermen may see fish acting differently than they’re used to — and they may see fewer fish where they usually expect to find them.